Terephthalic acid is commercially produced by oxidation of paraxylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid is used in the production of polyester fibers, films, and resins and must be further treated to remove impurities formed as a result of the oxidation of paraxylene.
Terephthalic acid (TPA) is an intermediate in the production of polyesters for plastics and fiber applications. An example of a process for manufacturing TPA is shown in FIG. 1. Commercial processes for the manufacture of TPA are often based on the heavy-metal catalyzed oxidation of p-xylene, generally with a bromide promoter in an acetic acid solvent or the combinations of these can be known as the reaction mixture 405. Due to the limited solubility of TPA in acetic acid under practical oxidation conditions, a slurry of TPA crystals is usually formed in the oxidation zone 400 which comprises at least one oxidation reactor. Typically, the TPA oxidizer slurry 410 is withdrawn from the oxidation zone 400, and TPA solids 450 can then be separated from the oxidizer mother liquor 415 in a TPA solid liquid separation zone 430, using conventional solid-liquid separation techniques. The oxidizer mother liquor 415, which contains most of the catalyst and promoter used in the process, is recycled to the oxidation zone 400 comprising at least one reactor. Aside from the catalyst and promoter, the oxidizer mother liquor 415 also contains dissolved TPA and many by-products and impurities. These by-products and impurities arise partially from minor impurities present in the p-xylene feed stream. Other impurities arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other by-products result from competing side reactions formed as a result of the oxidation of p-xylene to terephthalic acid. Patents disclosing the production of terephthalic acid such as U.S. Pat. Nos. 4,158,738 and 3,996,271 are hereby incorporated by reference in their entirety to the extent that they do not contradict statements herein.
The TPA solids can undergo a solid-liquid separation wherein fresh solvent 420 is utilized to displace a major portion of the liquid component of the oxidizer mother liquor 415. After drying, the TPA solids are contaminated with impurities that were present in the oxidizer mother liquor 415 since these impurities may be incorporated into the TPA solids. Impurities are also present due to occlusions in the TPA crystal structure and due to incomplete removal of the oxidizer mother liquor 415 by the fresh solvent wash 420.
Many of the impurities in the oxidizer mother liquor 415 stream that is recycled are relatively inert to further oxidation. Such impurities include, for example, isophthalic acid, phthalic acid and trimellitic acid. Impurities, which may undergo further oxidation are also present, such as, for example, 4-carboxybenzaldehyde, p-toluic acid and p-tolualdehyde. Oxidation inert impurities tend to accumulate in the oxidizer mother liquor 415 upon recycle. The concentration of these inert impurities will increase in the oxidizer mother liquor 415 until an equilibrium is reached whereby the rate of removal of each impurity via the TPA product balances with the rate of formation and the rate of addition to the oxidation process. The normal level of impurities in commercial terephthalic acid makes it unsuitable for direct use in most polymer applications.
Conventionally, terephthalic acid has been purified either by conversion to a dimethyl ester or by dissolution in water with subsequent hydrogenation over standard hydrogenation catalysts. More recently, secondary oxidative treatments have been used to produce polymer-grade TPA. It is desirable to minimize the concentration of impurities in the mother liquor and thereby facilitate subsequent purification of TPA. In some cases, it is not possible to produce a purified, polymer-grade TPA unless some means for removing impurities from the oxidizer mother liquor 415 is utilized.
One technique for impurity removal from a recycle stream commonly used in the chemical processing industry is to draw out or “purge” some portion of the oxidizer mother liquor 415 that is recycled. Typically, the purge stream is simply disposed of or, if economically justified, subjected to various treatments to remove undesired impurities while recovering valuable components. One example is U.S. Pat. No. 4,939,297 herein incorporated by reference in their entirety to the extent that they do not contradict statements herein. The amount of purge required for control of impurities is process-dependent; however, a purge amount equal to 10-40%, hereafter known as oxidizer purge stream 101, of the total oxidizer mother liquor 415 stream that is recycled is usually sufficient to produce TPA adequate as feedstock for commercial polymer manufacture. However, in an embodiment of the invention a purge amount up to 100% could be used.
In the production of TPA, the percentage purge of the oxidizer mother liquor 415 necessary to maintain acceptable impurity concentrations, coupled with the economic value of the metal catalyst and solvent components in the oxidizer purge stream 101, make simple disposal of the oxidizer purge stream 101 economically unattractive. Thus, there is a need for a process that recovers a major portion of the valuable metal catalysts and acetic acid contained in the oxidizer purge stream 101 while removing a major portion of the impurities present in the oxidizer purge stream 101. The metal catalyst can be recovered in an active form suitable for reuse by direct recycling to the p-xylene oxidation step.
Disclosed is an invention to recover a wash filtrate 148 and mother liquor 147 from an oxidizer purge stream. The oxidizer purge stream 101 is sent to a separation zone and then the mother liquor/and or wash filtrate is recycled back to an oxidation reactor. In should be noted that the above described TPA process is for example only. The disclosed invention can apply to many different TPA processes that are capable of producing an oxidizer purge stream 101. Therefore, it should be noted that this invention does not just apply to the terephthalic acid process, but any process that produces an oxidizer purge stream 101 where recovery of a mother liquor 147 and wash filtrate 148 comprising a metal catalyst is needed.